Hafnium: the essentials

Most zirconium minerals contain 1 to 3% hafnium. Hafnium is a ductile metal with a brilliant silver lustre. Its properties are influenced considerably by the impurities of zirconium present. Of all the elements, zirconium and hafnium are two of the most difficult to separate. Hafnium is a Group 4 transition element.

Because hafnium has a good absorption cross section for thermal neutrons (almost 600 times that of zirconium), has excellent mechanical properties, and is extremely corrosion resistant, it is used for nuclear reactor control rods.

Hafnium carbide is the most refractory binary composition known, and the nitride is the most refractory metal nitride (m.p. 3310°C).

Hafnium: historical information

Hafnium was discovered by Dirk Coster and George Charles von Hevesy in 1923 at Denmark. Origin of name: from the Latin name "Hafnia" meaning "Copenhagen".

Hafnium was thought to be present in various zirconium minerals many years prior to its discovery, in 1923, which was credited to Dirk Coster and George Charles von Hevesey. It was finally identified in zircon (a zirconium ore) from Norway, by means of X-ray spectroscopic analysis. It was named in honour of the city in which the discovery was made. A number of earlier claims seem less likely.

Most zirconium minerals contain 1 to 3% hafnium and it is their chemical similarity which made their separation difficult. It was originally separated from zirconium by repeated and tedious recrystallization of the double ammonium or potassium fluorides.

Hafnium is never found as the free element in nature. It is found in the ore alvite [(Hf, Th, Zr)SiO4.xH2O] and thortveitite. Most zirconium minerals (zircon, baddelyite) contain 1 to 3% hafnium and in practice most hafnium is produced as a byproduct of zirconium production, presumably because it is necessary to remove the hafnium from the zirconium used in the nuclear industries.

Isolation

Isolation: hafnium extraction is always associated with its removal from zirconium as it is a contaminant of all zirconium minerals. Solvent extraction methods are used ot spearate the two metals but the process is not easy. These make use of the differential solubilities of the metal thiocyantes (thiocyanate is SCN-) in methyl isobutyl ketone.

Hafnium isotopes have several applications. Hf-180 is used for the production of the radioisotope Hf-181 while Hf-180 is used for the production of the radioisotope Ta-179, which has a medical application. The second isomer of Hf-178 (Hf-178m2) exhibits a very high excitation energy and it has been suggested for use in gamma ray lasers.